Thin films of ferroelectric materials and a method for preparing same

Abstract

Thin films of ferroelectric material with a high mole fraction of Pb(A2+1 / 3B5+2 / 3)O3 substantially in a perovskite phase, wherein A is zinc or a combination of zinc and magnesium, and B is a valence 5 element such as niobium or tantalum, have been prepared. Typically, the mole fraction of Pb(A2+1 / 3B5+2 / 3)O3 in the ferroelectric material is >0.7. The method for preparing the thin films of ferroelectric material comprises providing a precursor solution containing lead, A2+, and B5+; modifying the precursor solution by addition of a polymer species thereto; applying the modified precursor solution to a surface of a substrate and forming a coating thereon; and (d) subjecting the coating to a heat treatment and forming the film in the perovskite phase. Optimal results have been obtained with PEG200 as the polymer species.

Description

FIELD OF THE INVENTION [0001] The present invention relates to thin films of a ferroelectric material and a method of preparation therefor. In particular the present invention relates to thin films of ferroelectric material with a high mole fraction of Pb(Zn1 / 3Nb2 / 3)O3 in a perovskite phase. BACKGROUND OF THE INVENTION [0002] In recent years, perovskite oxide solid-solution systems based on ferroelectric Pb(Zn1 / 3Nb2 / 3)O3 (PZN), such as (1-x)Pb(Zn1 / 3Nb2 / 3)O3-xPbTiO3 (PZN-PT); have attracted great attention due to their extremely large electromechanical strain and high dielectric constant. A PZN-PT single crystal exhibits high electromechanical coupling coefficients (k33>90%), and high piezoelectric coefficients (d33>2200 pC / N), properties which are significantly superior to those of the most widely used piezoelectric Pb(Zr,Ti)O3 (PZT) materials. Therefore, the success in developing perovskite PZN-PT bulk single crystal is thought as the most significant breakthrough in piezoele...

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Benefits of technology

[0081] A further advantage of the present invention over the present technology is that the thickness of thin films of ferroelectric material could extend across the range from micrometers to nanometer, rather than being constrained to beyond several micrometers.

[0082] The thin films of ferroelectric materials of the present invention exhibit a high dielectric constant and excellent piezoelectric properties. For example, the thin film of 0.77(0.6PZN-0.4PMN)-0.23PT discussed in Example 5 exhibits strong ferrolectric characteristics. The dielectric constant and loss are 3497 ad 0.06 at 1 kHz, respectively, and the dilatation of the thin film is about 325 pm in thickness direction at 5V.

[0083] Accordingly, it is envisaged that the thin films of ferroelectric materials of the present invention will be employed in microelectronic devices and microelectromechanical systems (MEMS) devices, such as: (1) integrated piezoelectric micro-actuators for many diverse applications, including microfluidic, ink-jet printing, nano-positioning (for example for hard disk drive, nano-scale manipulation, and such like), micro ultrasonic devices, optical switchers and modulators, switchers for RF MEMS, actuators for bioMEMS; (2) high frequency ultrasonic transducers for medical imaging; (3) micro resonators for many device applications; (4) integrated piezoelectric sensors; and, (5) integrated thin film capacitors.

[0084] A thin film of ferroelectric material with a high mole fraction of Pb(A2+1 / 3B5+2 / 3)O3 substantially in the perovskite phase, wherein A is zinc or a combination of zinc and magnesium, and B is a valence 5 element, such as niobium or tantalum, is prepared according to the following method, which method comprises providing a precursor solution containing lead, A, and B; modifying the precursor solution by addition of a polymer species thereto; applying the modified precursor solution to a surface of a substrate and forming a coating thereon; and subjecting the coating to a heat treatment and forming the film in the perovskite phase.

[0085] A target composition for the desired ferroelectric material is selected. The target compositions for PZN-PT-based ferroelectric materials are typically (1-x)Pb(Zn1 / 3Nb2 / 3)O3-xPbTiO3 (0.05≦x≦0.3) and the target compositions for PZN-PMN-PT-based ferroelectric materials are typically (1-x)Pb((Zn(1-y)Mgy)1 / 3Nb2 / 3)O3-xPbTiO3 (x≦0.3 and 0≦y≦0.7).

[0086] The precursor solution for the target composition for the ferroelectric material is prepared by mixing a first solution containing substantially stoichiometric molar ratios of lead, A, and B to form Pb(A2+1 / 3B5+2 / 3)O3 and a second solution containing substantially stoichiometric molar ratios of lead and titanium to form PbTiO3.

Problems solved by technology

However, the preparation of useful perovskite PZN-based thin films is extremely challenging because of the poor stability of the perovskite phase relative to the pyrochlore phase for the PZN composition.

Furthermore, the perovskite crystallographic structure in the prepared films was not in a preferred orientation.

A randomly orientated PZN-PT film with a very low PZN composition does not demonstrate optimal characteristics for commercial applications for several reasons.

Further, although superior electromechanical performance properties are observed along certain crystallographic directions in bulk PZN-PT single crystals, PZN-PT bulk ceramic materials with random crystallographic orientation do not exhibit the same excellent performance properties.

Finally, while the perovskite phase may be stabilized in films having thicknesses beyond several micrometers, such films are too thick for many commercial applications.

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